Thermal stratification, mixis, and advective currents in the Parakrama Samudra Reservoir, Sri Lanka

Abstract

Thermal stratification in the northern basin of Parakrama Samudra (PSN) was investigated in diurnal cycles at a point as well as in horizontal surveys. The data were used to calculate Birgean mechanical work (Birge 1916) and derived values. Although similar temperature ranges were recorded (28.0°–34.5° C) for August/September 1979 and March 1980, fluctuations in mixing work (Eckel 1948) (up to 4 J m^-2and 20 J m^-2, respectively) were observed, which is in accordance with the changing water levels of 1.8 m and 3.8 m, respectively. Deviations of 1.7-2.8 J m^-2from mean potential energy can drive currents to equalize local inhomogeneities of the stratification. The main source of these inhomogeneities may be the stronger temperature effect of heating and cooling in the shallower areas. It obviously drives advective currents towards the deeper areas where it amplifies the apparent heat budget about three times. Losses of Birgean energy incurred by cooling (1.7–5.9 J m^-2in 1979,27.1 J m^-2in 1980) are transformed into convective energy. This leads to erosion processes in the order of a few millimetres of sediment depth daily and hence settled algae can be redistributed every night. The density effects caused by heat exchange are obviously the predominant factor in the mechanics of such shallow tropical lakes.